Metzger's lexical aids has a list of more than 25 second declension (OS/OU)
nouns that are either always feminine or can be feminine or masculine. The
most noteworthy one in the GNT than can be either is DIAKONOS. A word used
in the GNT often that is feminine is hODOS/OU, hH, the way, road. The names
of a number of cities are 2nd declension feminine nouns. They all take the
feminine article.